Q. Increase in velocity of water due to afflux?

Specific energy E_f in m per m³ of flow is the energy measured with respect to bottom and hence comprising potential head D and velocity head V²/2g also in m. Thus E_f equals summation of D and V² /2g. When bridge waterway is constricted by making the bridge narrower than the channel width, discharge intensity q increases at the bridge section. A line of constant specific head E_f in the subcritical flow region, it is seen that river discharge with a particular specific head can pass through the bridge with different discharge intensities q implying different constrictions. Bigger the constriction, higher the value of q and lesser the depth D.

The velocity head v² /2g correspondingly becomes higher. Such progressive constriction of bridge waterway and increase in discharge intensity is, however, possible only up to a limit which is reached at critical stage. If further constriction is caused, the available specific energy will be insufficient to pass the discharge and hence flow will head up on upstream side of the bridge to force the discharge through. This heading up then becomes apparent as afflux and its effect is felt as backwater over long reaches. On the down stream side the energy built up in formation of afflux is dissipated by generation of hydraulic jump. Conditions leading to afflux on upstream and jump on the downstream side of constricted waterway of a bridge are possible when the bed is nonscourable. When riverbed is sandy, increased velocities in the constriction develop bed scour. This condition is considered separately.